The more prevalent freight railcar construction in the United States includes what are known as three-piece trucks. Trucks are wheeled structures that ride on tracks and two such trucks are normally used beneath each railcar body, one truck at each end. The term "three-piece" refers to a truck that has two sideframes which are positioned parallel to the wheels and the rails, and to a single bolster which transversely spans the distance between the sideframes. The weight of the railcar is generally carried by a center plate connected at the lateral midpoint on each of the bolsters.
Each cast steel sideframe is usually a single casting comprised of an elongated lower tension member interconnected to an elongated top compression member which has pedestal jaws depending downwardly from each end. The jaws are adapted to receive wheeled axles which extend transversely between the spaced sideframes. A pair of longitudinally-spaced internal support columns vertically connect the top and bottom members together to form a bolster opening which receives the truck bolster. The bolster is typically constructed as single cast steel section and each end of the bolster extends into each of the sideframe bolster openings. Each end of the bolster is then supported by a spring group that rests on a horizontal extension plate projecting from the bottom tension member.
Railcar trucks operate in severe environments where the static loading can be significantly magnified, therefore, they must be structurally strong enough to support the car, its payload, and the weight of its own structure. The trucks themselves are heavy structural components which contribute to a substantial part of the total tare weight placed upon the rails. The maximum quantity of product that a shipper may place within a railcar will be directly affected by the weight of the car body, including the trucks themselves. Hence, any weight reduction that may be made in the truck components will be directly available for increasing the carrying capacity of the car.
The designers of the early cast steel trucks experimented with several types of cross sections in their quest to reduce sideframe weight, but were unable to develop a successful "open" cross-section. Modern cast steel sideframes of the current three-piece truck configuration, are rather heavy due to the sideframe designs requiring cross sections of either box or C-shape. Furthermore, producing these types of cross sections requires numerous cores in the casting mold, which increases production costs and complicates the pouring process by adding complex channels inside the mold which must be filled with molten metal.
Fabricated sideframes were later seen as a revolutionary light weight replacement for the cast sideframe, but the presence of welds were found to reduce fatigue life and structural integrity of the sideframe. As a result of the low service life for fabricated sideframes, interest in the cast steel sideframes continued.
A more recent problem hindering the development of lighter and stronger sideframes is the fact that structural re-development of a cast steel sideframe design is extremely expensive, and it requires the approval of the American Association of Railroads (AAR) before the new design can become field-operational. The AAR review and approval process can take months, even years; for a complex design change. Therefore, it is not surprising that innovation in the railroad industry has proceeded slowly in the freight car truck design area. In spite of these handicaps, new analytical tools and a genuine need to help the railroads reduce costs is now at hand. The great strides made in development of computer technology and advanced engineering analysis has allowed sideframe designers to challenge old sideframe design principles and to design new sideframe members which are stronger, yet actually lighter than past designs. These latest techniques have increased the focus of attention towards maximizing the carrying capacity of the car while reducing the energy consumption realized from weight reductions in the railcar components.
Recent sideframe developments have concentrated on structurally re-designing the sideframe from the closed and box-type of cross-section, into an open, and I-beam shaped cross-section. A challenging new sideframe design of this type is described in U.S. Pat. No. 5,410,968 issued May 2, 1995 and assigned to AMSTED Industries Incorporated, Chicago, Ill., co-owner of the present application. The sideframe of that application provides an integrally cast I-beam shaped, solid sideframe in which the upper and lower compression and tension members comprise the flanges of the I-beam, while a vertical web interconnects the flanges together. Although a portion of the web is removed to reduce weight, a substantial weight savings is realized from the solid component construction, as compared to an open, box-type sideframe. By directing molten metal only to critical stress areas of the sideframe, weight savings between 200-250 pounds per sideframe can be realized. The range of weight savings is a function of the tonnage rating of the truck, i.e., 100 ton or 125 ton. Besides the advantages of saving weight, the solid, yet "open" I-beam structure provides that all sideframe surfaces will be in open, plain view for easy inspection. Prior art box-like sideframes have inside surfaces that are never in plain view and can never be visually inspected. This "open" feature provided several production and quality-related advantages over prior art sideframes.
As previously mentioned, all new railroad component design changes must be officially tested, verified, and then approved by the AAR before ever being placed into actual field use. One shortfall has been discovered with the sideframe of U.S. Pat. No. 5,410,968 when subjected to the "official" AAR transverse test methods; namely, inconsistent test results which have subjected some of the sideframes to failure of the static AAR transverse load tests. Those in the art are familiar with the AAR method of transverse testing wherein the sideframe is layed flat on one of its sides (see FIG. 2) and is supported and elevated at each sideframe end, or pedestal jaw, by a respective stationary post (not shown). The posts are secured to the ground. A clamp 300 and a steel bar 400 is then connected to each of the sideframe pedestal jaws, such that the clamp and bar extend between each of the supporting posts; a dial indicator 500 is attached to the midpoint of the bar. A vertical, downward test load is applied to the midsection of the sideframe, causing it to deflect and the dial indicator measures the total amount of static deflection. Under the AAR standards, a limited amount of deflection is allowed. Because the steel bar is directly connected to the sideframe at each pedestal jaw, the AAR transverse loading arrangement is considered a "floating-zero" type of measuring method since the test equipment (steel bar and dial indicator) is effectively "floating" with respect to the deflection in the sideframe. However, railcar designers typically use a fixed or "ground-zero" transverse testing method which is essentially similar to the AAR test method, except that the dial indicator is attached in a stationary position on the ground and is not allowed to "float". It is felt that this method of measurement is more representative of the true deflection than the AAR floating method.
When a transverse test load was applied to the lightweight sideframe of U.S. Pat. No. 5,410,968, using the AAR test method; the distal ends of the sideframe were found to slightly twist in the same longitudinal direction as the test bar. This lateral twisting behavior is expected at the sideframe ends since an I-beam construction is inherently susceptible to twisting. However, the twisting movements of the sideframe ends cause twisting in the test bar itself, and hence twisting of the "floating" dial indicator. The non-stationary dial indicator arrangement was found to create inconsistent and unreliable test results, leading to occasional non-compliance with the AAR transverse test standards. It is important to note that during actual operating conditions, twisting of the distal sideframe ends will not be as pronounced as during the AAR transverse tests since the axles will secure the sideframe ends against such movement and since this type of movement only occurs during truck curving or high speed truck hunting. Moreover, it should also be clarified that when the same transverse tests were performed using the "ground-zero" measuring methods, the sideframes easily satisfied all of the AAR transverse static load test standards. Even though the ground-zero test is widely accepted and used within the industry during in-house testing, the AAR transverse test methods currently control. Therefore, in order for the above-mentioned sideframe to become fully sanctioned according to AAR methods and standards, it was realized a lateral sideframe structure which could prevent the twisting of the "floating" dial indicator was needed.